[NTG-context] Changing font for math function

[Aditya Mahajan]

> And in my opinion, names for operators and subscripts should all be
> upright, that is adaptative to the surrounding TEXT STYLE (\rm, \ss and
> \tt) BUT not adaptative to the surrounding TEXT ALTERNATIVE or MATH
> ALTERNATIVE (\it, \bi, \sl, \bs) and stay in the normal alternative \tf
> (upright).

This behavior can be achieved using

\setupmathematics[functionstyle=normal]

Aditya